Early stage growth mechanisms of CdTe thin films deposited by close space sublimation for solar cells

The early stage growth mechanisms of sublimation-grown thin-film polycrystalline CdTe are evaluated by growth interrupts and ex-situ SEM/AFM analysis for growth under 100 Torr of inert gas. Development of island size, density and coverage demonstrates that growth proceeds by island nucleation, island growth and density increase, followed by coalescence, channel formation and secondary nucleation. Addition of material to the islands occurs partly by the ‘step-flow’ mechanism. Grains in the completed films are considered to arise from individual nuclei. Nucleation and coalescence models are used to explain the correlation between increased substrate temperature and increased CdTe grain size in sublimation deposited CdTe films.     

Recent progress on CdTe/CdS thin film solar cells

The CdTe/CdS thin film solar cell is the most suitable to be fabricated on the form of thin films. The processes used to make all the films, which compose the cell, are quite simple and fast. An efficiency of 16.5% has been reached on laboratory scale and modules of 0.6 × 1.2 m² with efficiency larger than 8% are now fabricated and commercialized. A strong contribution to the development of this type of solar cells has been given by the Parma University group with the discovery of a new ohmic back contact for CdTe which is very stable in respect to any other ohmic contact used for CdTe, and by the development of a new all dry process to make the cell. An efficiency of 15.8% has been recently obtained on a 10⁻⁴ m² soda-lime glass without using any copper or any other metal of the first group of the periodic table of the elements at the back contact.     

Photovoltaic properties of CdTe solar cells fabricated by close spaced sublimation with screen printed CdTe sources

Screen printed CdTe layer was employed as a source for close spaced sublimation process. By controlling the initial mixture of screen printed source, the Cd content in the CdTe film varied from 49.5% to 48%. The solar efficiency of CBD CdS/CdTe cell strongly depended on the stoichiometry of CdTe films due the bulk resistivity change ranging from 2 × 10⁶ to 3 × 10⁴Ω cm. By comparing with the dark forward current of screen printed CdS/CdTe cell, it is suggested that in the CBD CdS/CdTe cell the grain size of CBD US layer should be increased to reduce the interface leakage current at low forward bias.     

Physical basis for the design of CdS/CdTe thin film solar cells

Solar cells based on polycrystalline semiconductor thin films have great potential for decreasing the cost of photovoltaic energy. However, this kind of solar cells has characteristics very different from those fabricated on crystalline silicon for which the carrier-transport and behavior is clearly known. Instead, for hetero-junction solar cells made on less known polycrystalline materials the design is almost empirical. In this work, several physical aspects related to the behavior of polycrystalline thin film solar cells will be discussed, and some considerations for an adequate design of this kind of solar cells will be made. For example, the recombination at the grain boundaries and its influence on the short circuit current as a function of the crystallite sizes on the active material is considered. Based on this, the appropriate thickness of each layer and their resistivity will be discussed. As an example, these considerations will be applied to CdS/CdTe heterojunction solar cells, taking into account typical properties of CdTe thin films used for solar cells.     

Diamond/CdTe: a new inverted heterojunction CdTe thin film solar cell

Recently the concept of the inverted photovoltaic cell has become more attainable as a practical cell. This thin film cell consists of a p–n heterojunction in which the window layer is p-type and the absorber layer is n-type. The feasibility of a new inverted p–n heterojunction p-diamond/n-CdTe solar cell has been demonstrated. The non-optimized solar cell structure grown on semi-transparent p-diamond yielded an open circuit voltage of 0.23 V and a short circuit current of 1.54 mA/cm² when illuminated.     

15% Efficiency CdS/CdTe thin film solar cells using CdS layers doped with metal organic compounds

For improving the photovoltaic performance of CdS/CdTe thin film solar cells, the CdS window layer is one of the most crucial factors. Here we demonstrate the photovoltaic performances of the low-environmental-load CdS/CdTe solar cell employing the CdS layer doped with various metal organic (MO) compounds, i.e., (CH₃)₂SnCl₂, (C₆H₅)₃GeCl, (CH₃CO₂)₃In, [(C₂H₅)₂NCS₂]₂Zn. Due to the MO doping, the degree of (1 1 1) preferential orientation of CdTe on the CdS layer is improved remarkably, influencing the increases in V_oc and F.F. Being almost independent of the kind of the MO compounds, the short circuit current increases due to increasing optical transmittance of the MO-doped CdS layers. As a result, utilizing MO-doped CdS, we have achieved the conversion efficiency of 15.1%.     

Metal/CdTe/CdS/Cd1−xZnxS/TCO/glass: A new CdTe thin film solar cell structure

The potential of CdTe/CdS/Cd_1−xZn_xS structure as an alternative to CdTe/CdS structure in photovoltaic application has been demonstrated. The unoptimized solar cell structure grown on transparent conducting oxide coated soda lime glass of 3 mm thickness with no antireflection coating yielded a 10% efficiency. This efficiency is the highest ever recorded in any Cd_1−xZn_xS film containing CdTe solar cells.     

Effect of electron irradiation on the properties of CdTe/CdS solar cells

Polycrystalline CdTe/CdS solar cells are used in space, as well as terrestrial, applications. The results of the studies on the effect of 8 MeV electron irradiation on p-CdTe/n-CdS thin film solar cells prepared by radio frequency (RF) sputtering are presented in this article. Solar cell parameters like short circuit current (I_sc), open circuit voltage (V_oc), fill factor (FF), conversion efficiency (η), saturation current (I_s) and ideality factor (n) have been considered. CdTe thin film solar cells exhibit good stability against electron irradiation up to 100 kGy.     

Influence of ITO surface modification on the growth of CdS and on the performance of CdS/CdTe solar cells

We treated the surface of indium–tin oxide (ITO) substrates in two ways, (i) coating of thin insulating ITO layer or (ii) irradiation of the surface with accelerated ions, and investigated the change in sheet resistance (R_sh) and the water-contact angle (WCA). R_sh increased with the thickness of the insulating ITO layer or with the ion dose. WCA dropped as a result of the surface treatment to <15°. The microstructure, the surface morphology, the optical transmittance, and the stoichiometry of CdS improved with the surface treatment. CdS/CdTe solar cells showed a better performance as a result of ITO surface treatment.     

Chalcopyrite thin film solar cells by electrodeposition

This paper reviews the state of the art in using electrodeposition to prepare chalcopyrite absorber layers in thin film solar cells. Most of the studies deal with the direct preparation of Cu(In,Ga)Se₂ films, and show that the introduction of gallium in the films is now becoming possible from single bath containing all the elements. Electrodeposition can also be used to form precursor films with stacked layer structures, of pure elements or of combinations with binary or even ternary films. Thermal annealing treatments are of dramatic importance to provide suitable electronic quality to the layers. They are often done in the presence of a chalcogen (selenium or sulfur) over pressure and there is a tendency to use rapid thermal processes. Less studies are devoted to complete solar cell formation. Significant progresses have been made in the recent period with several groups achieving cell efficiencies around 8–10% on different substrates. A record efficiency of 11.3% is reported for a cell with an absorber presenting a band gap of 1.47 eV. First results on the manufacturability of the corresponding process to large areas are presented.     

Advances in monocrystalline Si thin film solar cells by layer transfer

The transfer of monocrystalline Si films enables the fabrication of efficient thin film solar cells on glass or plastic foils. Chemical vapor deposition serves to epitaxially deposit Si on quasi-monocrystalline Si films obtained from thermal crystallization of a double-layer porous Si film on a Si wafer. A separation layer that forms during this crystallization process allows one to separate the epitaxial layer on top of the quasi-monocrystalline film from the starting Si wafer after solar cell processing. Independently confirmed thin film solar cell efficiencies are 15.4% and 16.6% for thin film solar cells transferred to a glass superstrate with a total Si film thickness of 24.5 and 46.5 μm, respectively, and a cell area of 4 cm². Device simulations indicate an efficiency potential above 20%.     

Electro-optical characterization and modeling of thin film CdS–CdTe heterojunction solar cells

Optoelectronic characteristics of thin film CdTe–CdS solar cells fabricated at four different laboratories were measured and analyzed. Current versus voltage measurements revealed that, under one sun illumination, tunneling was the dominant current flow mechanism in all cells. Tunneling was also the dominant current flow mechanism in the dark for all types except P3 which exhibited a generation-recombination type current flow process in the dark. A theoretical model involving bulk traps in CdTe and a charged thin layer (T-layer) near the junction under forward bias and/or illumination was developed. The model is able to explain all significant features in the experimental results obtained from current versus voltage, and capacitance.     

Stability of CdTe/CdS thin-film solar cells

The recent literature regarding the stability of CdTe/CdS photovoltaic cells (as distinguished from modules) is reviewed. Particular emphasis is given to the role of Cu as a major factor that can limit the stability of these devices. Cu is often added to improve the ohmic contact to p-CdTe and the overall cell photovoltaic performance. This may be due to the formation of a Cu₂Te/CdTe back contact. Excess Cu also enhances the instability of devices when under stress. The Cu, as Cu⁺, from either Cu₂Te or other sources, diffuses via grain boundaries to the CdTe/CdS active junction. Recent experimental data indicate that Cu, Cl and other diffusing species reach (and accumulate at) the CdS layer, which may not be expected on the basis of bulk diffusion. These observations may be factors in cell behavior and degradation, for which new mechanisms are suggested and areas for future study are highlighted. Other possible Cu-related degradation mechanisms, as well as some non-Cu-related issues for cell stability are discussed.     

Back electrode formation for poly-Si thin film solar cells on glass having AIC-grown seeding layer

Various conductive materials (Al, Mo and TiN) were deposited onto glass substrates to evaluate whether poly-Si seed layers can be formed on such substrates by means of Al-induced crystallisation (AIC) of a-Si at low temperature around 450°C. The material located between the glass and the poly-Si film serves as the back electrode of a substrate-type thin-film solar cell configuration. The outcome of the investigation is that Mo is found to be not compatible with the AIC process. In contrast, Al and TiN showed moderate to good compatibility. TiN is the only viable choice for high-temperature applications (>540°C). Al has satisfactory back electrode properties whereas TiN has a medium high resistivity (120 μΩ cm) and an estimated low back reflectance at the near-infrared wavelengths critical for light trapping.     

Hybrid organic–inorganic solar cells: Case of the all thin film PMeT(Y)/CdS(X) junctions

Hybrid organic–inorganic all thin film photovoltaic junctions PMeT(Y)/CdS(X) were investigated, where PMeT(Y) is the conducting polymer poly(3-methylthiophene) doped with various anions Y=CF₃SO₃⁻, ClO₄⁻, BF₄⁻, PF₆⁻, and CdS(X) cadmium sulfide doped with various elements X=Cu, Ni, Al, As and Sb. CdS(X) films were spray deposited on conducting and transparent indium-tin oxide (ITO) glass, and PMeT(Y) films were electrodeposited onto the CdS(X) film to form the junction. The electrochemical investigation of the mechanism of electrodeposition and growth of the PMeT(Y) films by means of chronoamperometry, and of the charge transfer behavior of the PMeT(Y)/CdS(X) junctions by means of cyclic voltammetry revealed a strong effect of the nature of Y and X. The same strong effect of Y was also found with PMeT(Y)/ITO junctions, and may have some generality. It was showed that the best quality of contact between the organic and inorganic phases, from an electrochemical viewpoint and in solution, was obtained with the junction, pointing towards a similarly better photovoltaic performance in solid state. This was indeed the case, and it was found that higher short-circuit current, open-circuit photovoltage and energy conversion efficiency, approaching 4%, could be obtained with this junction.     

Influence of CdS window layer on 2-μm thick CdS/CdTe thin film solar cells

Influence of the CdS window layer on the PV performances of 2-μm thick CdS/CdTe solar cells has been studied as a function of the CdS thickness, d_CdS. With a reduction of d_CdS from 114 to 95 nm, J_SC increases due to an increase in blue response. While, at d_CdS<85 nm, the conversion efficiency largely decreases due to a decrease in V_OC and FF. The deterioration of the crystallinity of CdTe due to a decrease in the sulfur composition x of the CdTe_1−xS_x mixed-crystal layer is concluded to be the most possible mechanism for the large decreases in V_OC and FF.     

Effects of grain boundaries on cell performance of poly-silicon thin film solar cells by 2-D simulation

The effect of grain boundaries on the performance of poly-Si thin film solar cells was studied theoretically using a 2-D simulation assuming the presence of either rectangular-shaped or graded width grain boundaries in the i-layer of p/i/n structure of solar cells. The grain boundary had an adverse effect mainly on V_oc. J_sc gradually increased and saturated with increasing solar cell thickness in cells without grain boundaries, whereas it reached a maximum for an i-layer thickness of 5 μm in polycrystalline silicon cells. The calculation using the graded width model showed that the efficiency of the p⁺/p⁻/n⁺ structure was better than that of the p⁺/n⁻/n⁺ structure. A slight p-type doping of the i-layer was found to be effective in improving cell performance.     

Chemical bath deposition of Cds buffer layer for GIGS solar cells

We investigated the chemical bath deposition of US thin flims on the Cu(In,Ga)Se₂ (GIGS) absorber layers and glasses. The process of the chemical bath deposition of US layer affected the performance of the CIGS solar cells. The CdS layers were deposited on the CIGS film from CdI₂, thiourea (NH₂CSNHn₂) and ammonia solutions. The influence of pH on the chemical bath deposition process was studied. The surfaces of the US films were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The compositions of the obtained CdS layers were analyzed by Auger electron spectroscopy (AES). The performance of the CIGS solar cells was discussed on the basis of the characteristics of the chemical bath deposited layer. We have successfully fabricated a high-efficiency CIGS solar cell with an efficiency of 17% using a US layer with stoichiometric composition.     

New materials and deposition techniques for highly efficient silicon thin film solar cells

This paper reviews recent efforts to provide the scientific and technological basis for cost-effective and highly efficient thin film solar modules based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon. Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes. Necessary prerequisite was the detailed knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. High rate deposition using plasma enhanced chemical vapour deposition at 13.56 MHz plasma excitation frequency was developed for μc-Si:H solar cells yielding efficiencies of 8.1% and 7.5% at deposition rates of 5 and 9 Å/s, respectively. These μc-Si:H solar cells were successfully up-scaled to a substrate area of 30×30 cm² and applied in a-Si:H/μc-Si:H tandem cells showing initial test cell efficiencies up to 11.9%.     

Efficiency potential of thin film polycrystalline silicon solar cells by silane-gas-free process using aluminum-induced-crystallization

Analyzing the performance of thin film polycrystalline silicon solar cells fabricated by silane-gas-free process including the aluminum-induced-crystallization technique by using the device simulation program “PC1D”, we have estimated the efficiency of them. In addition, we have discussed the issues to make the silane-gas-free process practical. In the cell fabrication by silane-gas-free process, segregation of impurity atoms at the grain boundaries of the Si film is one of the serious problems. By suppressing the impurity inclusion and optimizing the cell parameters, the simulated efficiency is to be about 13% in single-junction cells.